1. Field of the Invention
The present invention relates to a method of producing oxide ceramic layers on barrier layer-forming metals or their alloys by plasma-chemical anodic oxidation in aqueous organic electrolytes, wherein the oxide ceramic layer may be further modified for specific applications. The present invention further relates to articles produced by the method.
2. Description of the Related Art
In aqueous electrolytes, the anodic oxidation described above is a gas/solid reaction under plasma conditions in which the high energy input at the base point of the discharge column produces liquid metal on the anode which forms with the activated oxygen a temporarily molten oxide. The layer formation is effected by anodes. The spark discharge is preceded by a forming process (P. Kurze; Dechema-Monographien Volume 121-VCH Verlagsgesellschaft 1990, pages 167-180 with additional literature references). The electrolytes are selected in such a way that their positive properties are combined and high-quality anodically produced oxide ceramic layers are formed on aluminum. By combining different salts, higher salt concentrations can be achieved in the electrolytic bath and, thus, higher viscosities can be achieved. Such high viscosity electrolytes have a high thermal capacity, they stabilize the oxygen film formed on the anode and, thus, they ensure a uniform oxide layer formation (DD-WP 142 360).
Because of the pattern of the current density/potential curves for the anodic spark discharge, three distinct portions can be distinguished. i.e. the Faraday portion, the spark discharge portion and the arc discharge portion, see P. Kurze mentioned above.
A barrier layer is naturally found on the metal or the metal alloy. By increasing the voltage of the anodically poled metal, the barrier layer increases. Consequently, a partial oxygen plasma which forms the oxide ceramic layer is created at the phase boundary metal/gas/electrolyte. The metal ion in the oxide ceramic layer is derived from the metal and the oxygen from the anodic reaction in the aqueous electrolyte. The oxide ceramic is liquid at the determined plasma temperatures of approximately 7,000.degree. Kelvin. Toward the side of the metal, the time is sufficient for allowing the melted oxide ceramic to properly contract and, thus, form a sintered oxide ceramic layer which has few pores. Toward the side of the electrolyte, the melted oxide ceramic is quickly cooled by the electrolyte and the gases which are still flowing away, particularly oxygen and water vapor, leave an oxide ceramic layer having a wide-mesh linked capillary system. Pore diameters of 0.1 .mu.m to 30 .mu.m were determined by examinations using electron scan microscopes (Wirtz, G. P., et al., Materials and Manufacturing Processes, 1991, "Ceramic Coatings by Anodic Spark Deposition," 6(1):87-115, particularly FIG. 12).
DE-A-2 902 162 describes a method in which spark discharge during the anodizing process is utilized for manufacturing porous layers on aluminum intended for use in chromatography.
EP-A-280 886 describes the use of the anodic oxidation with spark discharge on Al, Ti, Ta, Nb, Zr and their alloys for manufacturing decorative layers on these metals.
The above-described methods make it possible only to manufacture ceramic layers having relatively small thicknesses of up to a maximum of 30 .mu.m which are insufficient for use as wear and corrosion protection layers.